Identification of intergenomic recombinations in unisexual salamanders of the genus Ambystoma by genomic in situ hybridization (GISH)

Abstract

Unisexual salamanders in the genus Ambystoma (Amphibia, Caudata) are endemic to eastern North America and are mostly all-female polyploids. Two to four of the bisexual species, A. laterale, A. jeffersonianum, A. texanum and A. tigrinum, contribute to the nuclear genome of unisexuals and more than 20 combinations that range from diploid to pentaploid have been identified in this complex. Because the karyotypes of the four bisexual species are similar, homologous and homoeologous chromosomes in the unisexuals can not be distinguished by conventional or banded karyotypes. We chose two widespread unisexual genomic combinations (A.laterale–2 jeffersonianum [or LJJ] and A. 2 laterale–jeffersonianum [or LLJ]) and employed genomic in situ hybridization (GISH) to identify the genomes in these unisexuals. Under optimum conditions, GISH reliably distinguishes the respective chromosomes attributed to both A.laterale and A. jeffersonianum. Of four populations examined, two were found to have independently evolved homoeologous recombinants that persist in both LJJ and LLJ individuals. Our results refute the previous hypothesis of clonal integrity and independent evolution of the genome combinations in these unisexuals. Our data provide evidence for intergenomic interactions between maternal chromosomes during meiosis in unisexuals and help to explain previously observed non-homologous bivalents and/or quadrivalents among lampbrush chromosomes that were possibly initiated by partial homosequential pairing among the homo(eo)logues. To explore the utility of GISH in other members of the complex, probes developed from A. laterale were also applied to unisexuals that contained A. tigrinum and A. texanum genomes. GISH is an effective tool that can be used to identify and to quantify genomic constituents and to investigate intergenomic interactions in unisexual salamanders. GISH also has potential application to examine possible genomic evolution in other unisexuals.